The present invention relates to an automatic transmission having a torque converter with a lockup clutch for directly coupling input and output shafts of the torque converter in response to the application of a lockup signal, and more particularly to a lockup clutch control system for such an automatic transmission, the system being capable of preventing the issuance of a lockup signal in the event of detection of a certain rate of deceleration.
Automatic transmission incorporate a torque converter for taking up variations in the torque transmitted from an engine and transmitting an amplified engine torque to a rear transmission mechanism. The torque converter has input and output shafts which are not mechanically coupled with each other and hence are subject to slipping. Therefore, the torque converter has a poor efficiency of power transmission. To cope with this problem, there has been put to use an automatic transmission having a lockup clutch for directly coupling torque converter input and output shafts at relatively high vehicle speeds at which no increased torque is necessary and engine torque variations cause no problem.
The automatic transmission with such a lockup clutch is disadvantageous in that when the road is slippery at the time the vehicle is braked or when the vehicle is abruptly braked, the wheels are locked against rotation. If the lockup clutch is connected or locked up at this time, then the lockup clutch is not released soon and the engine is stopped, a condition which is dangerous in the driving of the vehicle.